Soil takes longer to cool than the air does, so in the earlier parts of winter the air may be well below freezing, while the ground is still thawed and water flows freely through it. If the air is sufficiently cold, water at or near the soil surface may freeze. Water expands when it freezes and this ice expands into the empty space above. If the air is cold enough, itÂ continues to freeze water near the surface. The expanding ice pushes ever upwards, drawing more water up through the soil by capillary action. The process continues, with endless tiny spires being thrust through the soil. Above the soil the spires may fuse together, forming small chunks 2-3″ thick.
Sadly, once the soil freezes completely, the process ceases. You”re most likely to encounter needle ice in late fall before the soil freezes and again in early spring as it thaws. It’s also common in more mild winters (and in those odd spells where temperatures soar up to nearly 60). Essentially anytime the upper layers of soil have liquid water and when air temperatures are well below freezing you can find needle ice on a trail near you.